1. Field of the Invention
The present invention relates to optical semiconductor devices, and more particularly, to a communications semiconductor laser, an optical modulator, both for use as an optical-fiber transmission light source, and an electro-absorption light source that is an integrated unit of both.
2. Description of the Related Art
The proliferation of the Internet on a worldwide scale is increasing the data traffic of optical communications networks each year. In response to this increase, the semiconductor-based transmission light sources and optical modulators that are among the key devices in optical-fiber communications networks, or electro-absorption light sources that are each an integrated unit of the former two types of devices are required to ensure even higher performance in terms of, for example, speed and electric power consumption. Along with these demands for higher performance, optical devices with active layers formed of InGaAlAs-based materials have been actively developed in recent years to replace the conventional devices that use InGaAsP-based materials. The multi-quantum-well (MQW) structures that use InGaAlAs-based materials are preferable to the structures that use InGaAsP-based materials. As shown in FIG. 4, the value of ΔEc:ΔEv, the rate of the discontinuity of the conduction band between each quantum well layer and barrier layer in an MQW structure, to the discontinuity of the valence band in the MQW structure, is 4:6 in InGaAsP-MQW, whereas the value of ΔEc:ΔEv in InGaAlAs-MQW is 7:3. In the InGaAlAs-MQW structure, therefore, electrons smaller than holes in effective mass are liable to be confined within the quantum well layers more easily and holes larger than electrons in effective mass are prone to be distributed more evenly over each quantum well layer within the MQW structure. Reference number 15 in FIG. 4 denotes a quantum well layer of the InGaAlAs-MQW structure, 16 a barrier layer thereof, 17 a quantum well layer of the InGaAsP-MQW structure, and 18 a barrier layer thereof. Since the effective mass of electrons in a semiconductor is less than one-sixth of that of holes, it can be seen that in terms of band structure, the InGaAlAs-MQW structure is preferable to the InGaAsP-MQW structure.
The preferableness of the InGaAlAs-MQW structure in terms of band structure, however, is conditional upon no application of strain to the quantum well layers or the barrier layers. The improvement of a differential gain is required for faster operation, and strain is applied to the quantum well layers as one method of increasing a differential gain. Strain application to the semiconductor deforms the valence band structure and separates the heavy holes and light holes that have been degenerate. Consequently, the influence of valence band mixing diminishes and effective mass decreases. Applying this method to the quantum well layers, therefore, increases an optical gain and a differential gain. Further increasing the number of quantum wells correspondingly improves the differential gain. Dislocation occurs in the semiconductor crystal if the number of quantum wells is increased under the state where strain is applied only to the quantum well layers. For this reason, it is very effective to adopt a strain-compensated structure in which strain is applied to barrier layers in a direction reverse to that of strain application to quantum well layers. Hereinafter, the compressive strain applied to the wafer is expressed with a plus (+) sign, and the tensile strain applied is expressed with a minus (−) sign.
The application of strain, however, shifts the ‘end of band’ of the semiconductor, resulting in a ratio different from the ΔEc:ΔEv ratio obtained under no strain. For example, in the quantum well structure having a 1.5%-strained quantum well layer and a −0.55%-strained barrier layer with respect to an InP wafer, ΔEc:ΔEv is almost 5.9:4.1. Also, in the quantum well structure having a −1.2%-strained quantum well layer and a 0.55%-strained barrier layer, ΔEc:ΔEv is almost 6.4:3.6. These values indicate that an InGaAlAs-based quantum structure in which strain is applied to quantum well layers in order to enhance performance is not always preferable in terms of band structure. In the conduction band, a carrier overflow of electrons occurs, and in the valence band, holes are liable to exist unevenly within the MQW structure. This is due to the fact that the ΔEc and ΔEv values that denote band discontinuity are determined by the composition ratio and strain level of InxGayAl(1-x-y)As, and more particularly, due to the fact that it is almost impossible to change ΔEc and ΔEv independently. Therefore, if the bandgap of the barrier layer (shortening the composition wavelength) is increased in an attempt to suppress the carrier overflow of electrons in the conduction band, both ΔEc and ΔEv will increase and holes will be easier to exist unevenly over each quantum well layer in the valence band. As a result, overall laser characteristics inclusive of a differential gain will not improve.
Furthermore, experimental results by the present inventors obviously indicate that increases in ΔEv of InGaAlAs-MQW lasers increases device resistance. FIG. 5 shows the relationship between the threshold current (Ith) data that the inventors measured in InGaAlAs-MQW ridge waveguide lasers, and the device resistance value standardized with mesa stripe width (Wa). Changing the threshold current by varying cavity loss in terms of edge reflectivity, for example, changes device resistance, even for the same waveguide structure including the same active layers. The symbol Δ in the figure denotes a laser in which a barrier layer was created with an InGaAlAs composition wavelength of 0.92 μm (bandgap: 1.348 eV), and the symbol ◯ denotes a laser in which a barrier layer was created with an InGaAlAs composition wavelength of 1.0 μm (bandgap: 1.239 eV). Both laser structures are the same, except for the composition wavelength of the barrier layer. As can be seen from the figure, the device resistances with the composition wavelength of 1.0 μm are lower than those with the composition wavelength of 0.92 μm. In consideration of the ratio between electrons and holes in terms of effective mass, and the ratio between ΔEc and ΔEv, the reason for the reduction in the resistance is attributable to the magnitude of ΔEv in the valence band. In other words, ΔEv of the barrier layer with the 0.92-μm composition wavelength, with respect to that of the quantum well layer, is 156 meV, whereas ΔEv of the barrier layer with the 1.0-μm composition wavelength is 123 meV. This difference appears as the difference in device resistance.
For these reasons, with the conventional technology, because of ΔEc and ΔEv being independently uncontrollable, it has been very difficult to obtain a device that simultaneously achieves the improvement of a differential gain and the improvement of laser characteristics, such as reduction in device resistance.
Other problems associated with the relationship between ΔEc and ΔEv also occur in electro-absorption optical modulators. If ΔEv is larger, the event of pile-up occurs when a reverse voltage is applied at an eletcro-absorption optical modulator. Holes cannot be removed from the MQW structure in the pile-up phenomenon. This results in optical signals being deteriorated by the particular pattern of the optical signals or in optical-fiber long-distance transmission being impeded by dynamic fluctuations in optical wavelength, called “chirping.” Decrease in the bandgap of the barrier layer (extending the composition wavelength) reduces ΔEv and thus suppresses the pile-up event. However, since ΔEc is also reduced at the same time, the resulting quantum effect in the conduction band is diminished and this blurs the absorption edge of the semiconductor bandgap. Accordingly, the problem occurs in that even when the reverse voltage is not applied, i.e., even at a ‘1’ level, light is absorbed and its extinction ratio deteriorates. This problem is also due to the independent uncontrollability of ΔEc and ΔEv.
A semiconductor multilayer structure based on another conventional technology, and an optical control device using the semiconductor multilayer structure are disclosed in Japanese Patent Laid-open No. 2003-329988. According to this disclosure, a 1.8-nm-thick In0.8Ga0.2As film is used to form quantum well layers, and a 5.5-nm-thick AlAs0.55Sb0.45 or AlAs0.5Sb0.5 film is used to form barrier layers. This device utilizes the large ΔEc value of a conduction band to achieve high-speed optical switching based on the intersubband transition between the first level and second level in the conduction band. However, this technology is not used in regard to whether ΔEv and ΔEc are independently adjustable, and the improvement of a differential gain that is an object of the present invention is not described in the above disclosure. In addition, since the type of input signal is light, no description is given of reduction in device resistance. According to the above disclosure, ΔEv is at least 280 meV. In addition, all layers of the device, such as the cladding layers, except in quantum well layers, are not doped with the dopant because of its operating principles.